basidiospore a type of sexual spore formed externally on a basidium, 1-2 or 4 in number, typical of the basidiomycetes. It is haploid and form acrogenously on a sterigma and typically results form karyogamy and meosis. when it germinates it produces a primary mycelium, which then produces secondary and tertiary mycelia, from which the fruiting bodies are formed.